There is a general need to improve techniques for monitoring, and stratifying by risk, various chronic respiratory conditions such as chronic obstructive pulmonary disease (COPD), cystic fibrosis, non-cystic fibrosis bronchiectasis, and asthma as well as other conditions.
One approach to assessing the severity of respiratory conditions is to use a standardised clinical exercise test. Exercise capacity is a strong predictor for the risk of morbidity due to respiratory disease and can be used in Secondary care, Primary care and Social care settings to assess a patient. Often, however, it requires specialised testing facilities. There are several ways to determine Exercise Capacity and this typically is achieved by inducing a state of oxygen desaturation in the subject whilst they undergo some form of physical activity.
The most popular clinical exercise tests in order of increasing complexity are stair climbing, a 6 minute walk test (6MWT), a shuttle-walk test, detection of exercise-induced asthma, a cardiac stress test (e.g. Bruce protocol), and a cardiopulmonary exercise test. Assessment of exercise capacity has traditionally been done by asking patients subjective recollections about their capabilities. However, patients vary in their recollection and may over or underestimate of their true functional capacity.
The 6MWT test measures the distance that a patient can quickly walk on a flat, hard surface in a period of 6 minutes. It evaluates the global and integrated responses of all the patient's physiological systems involved during exercise, including the pulmonary and cardiovascular systems, systemic circulation, peripheral circulation, blood, neuromuscular units and muscle metabolism.
However, it does not provide specific information on the function of each of the different organs and systems involved in exercise or the mechanism of exercise limitation, as is possible with maximal cardiopulmonary exercise testing: The self-paced 6MWT assesses the submaximal level of functional capacity. Most patients do not achieve maximal exercise capacity during the 6MWT; instead, they choose their own intensity of exercise and are allowed to stop and rest during the test and this can affect the biometric parameters such as SpO2.
To compensate for this, some advocate that tests should have a fixed level of load (e.g. the Shuttle Test or the TChester). However, because most activities of daily living are performed at submaximal levels of exertion, the 6MWT may better reflect the functional exercise level for daily physical activities. The 6MWT is used as a one-time measure of functional status of patients, as well as a predictor of morbidity and mortality.
Formal cardiopulmonary exercise testing provides an overall assessment of the exercise response, an objective measurement of functional capacity and impairment, determination of the appropriate intensity needed to perform prolonged exercise, quantification of factors limiting exercise, and a definition of the underlying pathophysiologic mechanisms such as the contribution of different organ systems involved in exercise. But this requires a laboratory setting and skilled staff to perform the test.
Further, 6MWT does not determine peak oxygen uptake, diagnose the cause of dyspnea (breathlessness) on exertion, or evaluate the causes or mechanisms of exercise limitation but information provided by a 6MWT is generally considered to be complementary to cardiopulmonary exercise testing, not a replacement for it.
Despite the difference between these Exercise Capacity tests, there are good correlations between these and with disease prognosis. For example, there is good agreement between the 6MWT and peak oxygen uptake for patients with end-stage lung diseases. In some clinical situations, the 6MWT provides information that may be a better index of the patient's ability to perform daily activities than peak oxygen uptake; for example, 6MWD correlates better with indices for the quality of life and changes in 6MWT after therapeutic interventions correlate with subjective improvement in dyspnea.
In another approach, the shuttle-walking test is used which is similar to the 6MWT, but it uses an audio signal from a tape cassette to direct the walking pace of the patient back and forth on a 10 m course. The walking speed is increased every minute and the test ends when the patient cannot reach the turnaround point within the required time. The exercise performed is similar to a symptom limited, maximal, incremental treadmill test. An advantage of the shuttle walking test is that it has a better correlation with peak oxygen uptake than the 6MWT. Disadvantages include less validation, less widespread use, and more potential for cardiovascular failure while it is being performed.
The 6MWT should preferably be performed indoors, along a long, flat, straight, enclosed corridor with a hard surface. However finding a suitable location for a patient to undertake this in the home environment can be challenging. Before and after the 6MWT, the technician will typically measure several parameters, including the distance walked within 6 minutes at the patient's own pace (the 6MWD distance) and the levels of oxygenation of the patient's blood (SpO2) before and after (measuring the degree of desaturation). In practice, SpO2 is not used for constant monitoring during the exercise because of the known issues of movement artefacts and difficulties in interpreting the results, but this has been recommended (“Should oxyhaemoglobin saturation be monitored continuously during the 6-minute walk test?”, Fiore et al. (2011). Chronic Respiratory Disease. Vol 8. No. 3 181-184). While now well established and researched, the 6MWT requires facilities and skilled technicians to perform it. Nonetheless poor Q&A, different technicians, and inconsistencies create data of poor and unreliable quality.
More recently, Researchers have proposed an exercise capacity analysis in the form of a simple Sit-to-Stand (STS) test, as a way of inducing exercise-related deoygenation in patients with severe respiratory disease and this can be used in assessing progressive decline of lung function and providing a simple way to make a prognosis for the patient.
The test is simple: Count how many times a patient can move from the sitting position to the standing position and back down again in one minute. The theory and observation is that as lung function declines, the number of repetitions that a patient can undertake reduces. Every reduction in repetition count was associated with increased risk. Performance in the test is strongly associated with health and quality of life but not a predictor of exacerbation or flare-up of recurrent chest infections associated with long-term respiratory disease. Exercise capacity during the STS test is strongly associated with mortality—for example in one study the STS test was shown to be a stronger predictor of 2-year mortality than body mass index and an inability to undertake 19 reps or less held a high chance of mortality within 24 months.
Whatever the method used to induce oxygen desaturation during exercise, using just the distance walked or number of reps undertaken is still highly variable and these tests have to be performed carefully so that the patient invests the same level of effort on each test. This also makes it difficult to compare the level of risk between subjects.
There have been attempts to improve upon the data provided by such tests by measuring the degree of oxygen desaturation (SpO2) induced by the exercise (“Desaturation-distance ratio: a new concept for a functional assessment of interstitial lung disease”, Pimenta et al. (2010). Clinic Science 65(9):841-846). In practice, however, it is difficult for a user to perform a reliable test by themselves, at home.
Pulse Oximeters
Another difficulty which arises in this context is the difficulty of obtaining reliable oxygenation data from pulse oximetry. The signal detected by a pulse oximeter is small and easily affected by movement. Typically a finger clip is used to analyse pulsing arterial blood, but in practice such clips do not fit well. The measurement is very susceptible to errors resulting from, for example, selection of an inappropriately sized clip, poor clip placement, and any small motion by the patient which can disturb the position of the optical sensor arrangement within the clip with respect to the finger. Various attempts have been made to address this latter problem by incorporating an accelerometer in the pulse oximeter to detect patient motion so that data is only captured when the patient is stationary and is disregarded when the patient is moving. Such approaches are described in: US2010/0324384, US2010/0125188, and WO2010/103390.
For example, US2010/0324384, describes a wrist-worn transceiver incorporating an accelerometer with a wired connection to a plastic clip fastened with a strap to the base of a user's thumb, the clip comprising the sensor (LEDs and photodetector), the wrist-worn unit amplifying, filtering, digitising and processing the sensor signals to measure SpO2. However neither inherent resistance to movement, nor long-term use are considered.
Pulse oximetry measures blood oxygen (as oxyhaemoglobin) and relies upon the measurement of optical absorbance, at two or more wavelengths, of perfused tissue. Typically arterial oxygenation is distinguished from venous and other effects by sensing the varying portion of optical absorbance. This means, however, that the measurement tends to be very sensitive to movement. The problem is that the varying portion is typically a very small part of the overall absorbance (around 1%, but often significantly less). Small movements of the tissue and/or the sensor inevitably cause apparent changes in absorbance, often much larger than this level. Much development has been directed at solving this problem, both by design of the sensor and processing of the signals from the sensor.
Sensing is typically done by using a pair of light emitting diodes (LEDs) as sources of light—at wavelengths that show different absorbance for oxygenated and deoxygenated haemoglobin e.g. 660 nm and 940 nm, as is well known. The LEDs are positioned at one side of a section of perfused tissue and a single silicon diode photosensor is positioned on the opposite side to receive light passed through the section. This is “transmission” mode; an alternative is to place the emitters and sensor on the same side of the tissue—“reflection” mode, where diffuse reflection within the tissue allows measurement of optical absorbance to some depth within the tissue.
Various locations on the body are known as preferred locations for sensors for pulse oximetry. Most commonly fingertips are used because they are readily accessible, are well supplied with arterial flow and are of an appropriate thickness. Further, the anatomy is relatively simple and consistent between individuals. It is, however, somewhat inconvenient for subjects, particularly when bulky and/or heavy sensors are used. The wrist would be a more convenient location for subjects but it is much less suitable for pulse oximetry because the anatomy is very complex and the exact location of a sensor in relation to bones and tendons becomes a problem; small movements cause large changes in sensed signals. The large overall thickness of the wrist also means that very little light passes through, therefore needing more power to generate stronger illumination and making signal recovery much more difficult and unreliable. Other locations that are used include:                “Base” of the finger—weaker pulsatile signal compared to the fingertip and not much more convenient for the subject        Forehead—good for measuring brain oxygen but unacceptable for continuous monitoring in daily life        Ear lobe—small pulsatile signal and sensitive to compression that excludes arterial flow        Foot—good for infants in hospital but inconvenient and subject to much movement for continuous monitoring        
The fingertip, therefore, is the preferred location but improvements are desired. In particular:                Resistance of the sensing system to movement        Improve comfort and convenience for long-term monitoring.        
As previously mentioned, particularly in pulse oximeter systems for determining the exercise tolerance or capacity of a user, it is desirable to reduce the sensitivity of the pulse oximeter to user movement.
General
Background prior art can be found in: US2008/243393; US2003/073884; US2011/040197; US2011/224498; US2008/319327; U.S. Pat. Nos. 3,998,550; 4,167,331; 4,407,290; 4,773,422; US2008015424; US2007/0038050; WO2012/140559; US2014/0200420; EP0968681A; US2010/210924; U.S. Pat. Nos. 5,795,052; 5,800,349; “Measurement of Motion Activity during Ambulatory Using Pulse Oximeter and Triaxial Accelerometer” Young-Dong et al. Convergence and Hybrid Information Technology, 2008. ICCIT '08. Third International Conference on (Volume: 1); and “A wireless sensor network compatible wearable u-healthcare monitoring system using integrated ECG, accelerometer and SpO2”, Chung et al. Conf Proc IEEE Eng Med Biol Soc. 2008; 2008: 1529-32. doi: 10.1109/IEMBS.2008.4649460.
However there is a need for techniques to improve upon the health monitoring approaches which have been employed hitherto, in particular those using pulse oximetry.